1. Field of the Invention
The present invention relates to improved water-absorbent resin particles and the production thereof. More specifically, it relates to a process for producing water-absorbent resin particles comprising the step of treating water-absorbent resin particles with a certain kind of-a modified silicone oil, and to water-absorbent resin particles obtained by the process having a low hygroscopic blocking rate and dust level, and a good initial absorbency under load.
2. Description of the Prior Art
Conventional water-absorbent resins are broadly used in hygienic materials such as sanitary materials, disposable diapers or water retaining materials for soil. Examples of such water-absorbent resins include substantially water-insoluble water-absorbing crosslinked polymers such as crosslinked polyacrylic acid salts, self-crosslinked polyacrylic acid salts, crosslinked graft-copolymers of starch-acrylic acid salts, crosslinked copolymers of vinyl alcohol-acrylic acid salts, hydrolyzed crosslinked copolymers of acrylamide, neutralized crosslinked copolymers of isobutylene-maleic acid anhydride, and crosslinked carboxymethyl cellulose salts.
Although such water-absorbent resins can absorb a great amount of water or urine, the absorbing capability causes problems such as blocking of the water-absorbing resin particles by absorbing moisture, adhesion to the machine, or disabling the discharge of water-absorbing resin particles from the hopper or continuous regular supply to the machine in the process of storing or providing the water-absorbing resin particles to the machine for producing disposable diapers in conditions of high humidity.
As the means to improve the hygroscopic blocking rate, methods as the following examples 1-4 have been proposed:
1 a method of mixing hydrophobic silica fine particles having an average particle size of 0.05 .mu.m or smaller and a specific surface area of 50 m.sup.2 / g or greater to the water-absorbing resin particles as disclosed in the Japanese Patent Application Laid Open No. 133028/1981. PA1 2 a method of adding inorganic particles such as hydrated silicon dioxide, hydrated aluminum hydroxide, hydrated titanium hydroxide to the water-absorbing resin particles as disclosed in the Japanese Patent Application Laid Open No. 80459/1984. PA1 3 a method of treating the water-absorbent resin particles with a cationic surface active agent and further adding organic compound particles having a high melting point as disclosed in the Japanese Patent Application Laid Open No. 69854/1986. PA1 4 a method of mixing stearic acid and inorganic powders to the water-absorbing resin particles to form a film of stearic acid on the surface of the water-absorbing resin particles as disclosed in the Japanese Patent Application Laid Open No. 105064/1988.
Although the method 1 of adding a hydrophobic silica contributes to improving the hygroscopic blocking rate, it has problems such as reducing the initial absorbency under load and absorbing capacity due to the surface of the water-absorbing resin particles being covered with the hydrophobic silica, or generating much dust due to the mixed fine powders of silica. When the inorganic powders of the method 2 are not hydrophobic, although it does not reduce the initial absorbency under load or absorbing capacity, it has problems such as insufficient improvement of the hygroscopic blocking rate, generating much dust due to the mixed fine powders of inorganic silica like the method 1. The methods 3 and 4 of covering the surface of the water-absorbing resin particles with a hydrophobic organic compound having a high melting point or stearic acid improve the hygroscopic blocking rate to some extent but not sufficiently. Besides, they have problems such as reducing the initial absorbency under load and the absorbing capacity due to the organic compound having a high melting point or the stearic acid hindering the absorbing ability of the water-absorbing resins.